The present invention concerns an exhaust gas purifying equipment for removing a particulate in the exhaust gas of diesel engines.
Regulations on the exhaust gas of recent internal combustion engines, especially diesel engines are further reinforced year by year, and, in particular, it has become urgent to reduce particulate matter (PM, hereinafter) containing mainly carbon. The diesel particulate filter (DPF, hereinafter) is known as a equipment for removing this PM from the exhaust. In addition, the movement to enforce equipping a vehicle having a diesel engine with a DPF is becoming a reality.
However, as the collected PM would accumulate in the DPF to be provided on a vehicle having a diesel engine due to a repeated driving of the engine, it is necessary to regenerate the DPF by burning the collected PM. As this regeneration means, methods for burning PM by heating with an electric heater or a burner and so on are known.
In the case of adoption of this method for burning PM, because it is impossible to collect PM during the regeneration of the DPF by burning PM again, the system becomes larger in scale through collecting and burning alternately by disposing a plurality of DPFs in the exhaust passage. Moreover, securing the durability of the filter becomes problematic, as the temperature becomes high during the PM burning. For these reasons, this method has not come to be widely adopted.
With these problems as a background, in recent years a method of burning the collected PM continuously by means of active oxygen generated during the occlusion and reduction of NOx by supporting a NOx occlusion reduction type catalyst as shown in the Japanese Patent Publication No. 2600492 (p. 3 to p.6) with the DPF has appeared, and is known as an exhaust gas purifying equipment for a diesel engine. Besides, a method for a providing oxidation catalyst upstream of the DPF as shown in Patent Publication No. 3012249(p.2, p.3) is also known.
An exhaust gas purifying equipment for a diesel engine, equipped with a conventionally known continuous regeneration type diesel particulate filter (continuous regeneration type DPF, hereinafter) is shown in FIG. 12. Now, on the basis of FIG. 12, the exhaust gas purifying equipment for a diesel engine provided with a continuous regeneration type DPF shall be described.
An intake manifold 3 composing a part of an intake passage and an exhaust manifold 4 composing a part of an exhaust passage are provided with an engine main body 2 being composed by a cylinder block and cylinder head and so on. An intake pipe 5 composing a part of the intake passage is connected to the intake manifold 3, and an air cleaner 6 for cleaning suction intake air is arranged at the furthest upstream section of this intake pipe 5. The suction air cleaned by the air cleaner 6 passes through the intake pipe 5 and is delivered to the inside of a cylinder (not shown) through the intake manifold 3. An exhaust pipe 7 composing a part of the exhaust passage is connected to the aforementioned exhaust manifold 4. And the exhaust gas produced in the cylinder is emitted through the exhaust manifold 4 and the exhaust pipe 7.
The illustrated diesel engine is provided with a turbocharger 8 for supercharging suction air. This turbocharger 8 has an exhaust turbine 81 arranged in the exhaust pipe 7 and an intake compressor 82 arranged in the intake pipe 5. In addition, the illustrated diesel engine comprises an exhaust gas recirculate (EGR hereinafter) passage 9 for connecting the exhaust pipe 7 at an upstream potion of the aforementioned exhaust turbine 81 and the intake pipe 5 at a downstream potion of the aforementioned intake compressor 82.
An EGR valve 11 is arranged in the EGR passage 9. This EGR valve 11 being provided with a negative pressure actuator connected, for example, to a negative pressure tank (not shown), the degree of valve-opening thereof(that is, the EGR rate) is controlled by controlling the negative pressure quantity to be supplied according to the driving state by control means 10 mentioned below.
As is well known, the EGR is an exhaust purifying means intended to reduce NOx by inputting suction air containing the recirculative exhaust gas being made to recirculate into the cylinder. The exhaust gas exhausts after combustion. About the connection of the EGR passage and the engine side, the EGR passage connects the intake passage with the exhaust passage in this example of the prior art, it is evident that the intake manifold composing a part of the intake passage can be replaced of the intake passage and that the exhaust manifold composing a part of the exhaust passage can be replace of the exhaust passage.
A continuous regeneration type DPF 12 having an oxidation catalyst 121, a DPF 122 and NOx catalyst 14 in that order from the upstream side, is arranged in the exhaust pipe 7 downstream of the aforementioned exhaust turbine 81.
As the oxidation catalyst 121, for example, those catalysts where a wash coat layer is formed by coating the surface of a carrier made of honeycomb-shaped cordierite or heat-resistant steel with active alumina or the like, this coat layer being made to support a catalyst active component made of rare metal such as platinum, palladium, or rhodium and so on, are used. At this oxidation catalyst 121, NO2 is produced by oxidation of NO in the exhaust gas and, at the same time, H2O and CO2 are produced by oxidation of HC and CO in the exhaust gas.
The DPF 122 is made of for example porous cordierite or silicon carbide. Alternatively, a honeycomb filter of the so-called wall-flow type or a fabric type filter is used for DPF 122. In the honeycomb filter, a number of cells are formed in parallel, and inlets and outlets of the cells are sealed alternately as a check pattern. And in the fabric type filter, ceramic fiber is wound around a stainless steel porous pipe in a number of layers. In this case this DPF 122 collects PM in the exhaust gas.
The same as the composition and components of the aforementioned oxidation catalyst 121 can be used as those of this NOx catalyst 14. Here, this NOx catalyst 14 reduces NOx such as NO and so on in the exhaust gas to N2 or H2O. Thus, the continuous regeneration type DPF 12 is composed of at least the oxidation catalyst 121 and DPF 122 as mentioned above. And NO in the exhaust gas is then oxidized to NO2 by the oxidation catalyst 121, and PM collected in the DPF 122 is oxidized and burned with NO2 flowing in the DPF 122 arranged downstream of the oxidation catalyst 121.
In this system, it is unnecessary to provide special heating means such as electric heater, burner and so on, because, at this time, PM burns at a low temperature equal or inferior to 400° C. In addition, this fact presents the advantage of making the whole system simple and compact, because PM is collected at the same time as producing continuously the combustion of PM at a low temperature.
The illustrated diesel engine comprises an engine speed detection sensor 15 for detecting the engine speed, an accelerator sensor 16 for detecting the accelerator pedal application amount (accelerator opening=ACL), an intake temperature sensor 17 for detecting the temperature of the intake air sucked into the cylinder, and a control means 10 for controlling the fuel injection quantity to be injected into the cylinder. The intake temperature sensor 17 is arranged in the intake manifold 3. The control means 10 controls the fuel injection quantity by the aforementioned EGR valve 11, a fuel injection unit (not shown) based on detection signals from the engine speed detection sensor 15, an accelerator sensor 16 and intake temperature sensor 17 and so on.
The control means 10 has a memory which stores the date of a so-called fuel injection quantity as shown in FIG. 15 for setting the fuel injection quantity taking the engine speed and the accelerator opening as parameters. And the control means 10 calculates the fuel injection quantity on the basis of detection signals from the engine speed detection sensor 15 and the accelerator sensor 16. Then the control means 10 corrects the basic fuel injection quantity based on the detection value of the intake temperature sensor 15, and calculates the final fuel injection quantity. It should be appreciated that the final fuel injection quantity can be corrected from time to time by referring not only to the intake temperature but also to various other parameters (atmospheric pressure, smoke limit injection quantity and so on).
The efficiency of the reaction for oxidizing NO to NO2 in the aforementioned oxidation catalyst 121, the so-called “transformation rate”, varies largely according to the catalyst temperature in catalysts under the present. For example, though a satisfactory oxidation reaction can be observed in the active area between 250° C. and 400° C., NO is not transformed satisfactorily to NO2 in the other areas. In other words, NO2 is not generated enough to oxidize PM.
FIG. 13 shows the exhaust quantity of CO2 that is generated by the oxidation burning of PM in relation to the exhaust temperature (the temperature of the exhaust gas) of the engine. Observing this, one can understand that PM burns actively and the filer is regenerated between 250° C. and 400° C. Adversely, PM burning, namely DPF regeneration, hardly occurs in the other temperature areas. In other words, PM arrives to be collected continuously by the DPF without regenerating the DPF. In a state such as where a great quantity of PM is accumulated, if PM burning occurs, the burning progresses in a moment to causing considerable deterioration of filter durability or other problems.
In the case of a diesel engine to be mounted on a vehicle, an engine speed and an engine load change every second according to the operating state, and the temperature of the exhaust gas discharged form the engine also changes according to the operating state. FIG. 14 shows the exhaust temperature area taking an engine speed and an engine load as the parameter. As it can be understood also from FIG. 14, when both the engine load and the engine speed are high or low, the catalyst temperature is out of the active temperature area (from 250° C. to 400° C.), therefore NO is not sufficiently oxidized to NO2 in the oxidation catalyst. Hence the PM collected by the DPF is does not burn sufficiently and consequently the PM collection efficiency of the filter also lowers. As a result, the filter itself becomes clogged early, or causing other unfavorable results. Furthermore, even if the exhaust temperature is within the catalyst active temperature area, in the case that the exhaust temperature is at the lower area, sometimes it results in being lowered below the active temperature area, because the heat of the exhaust gas is radiated to the atmosphere or others, in the course from the exhaust manifold to the oxidation catalyst.
And, especially in an environment such as a very cold land and a highland, it is a difficult to burn and remove the collected PM completely, in any operating range.
Though the aforementioned prior art has been described taking the continuous regeneration type DPF composed of an oxidation catalyst and a diesel particulate filter as an example, the same problem also occurs in a method for burning the collected PM continuously using the active oxygen generated through the occlusion and the reduction of NOx by supporting the NOx occlusion reduction type catalyst on the DPF, because the temperature area where the catalyst functions effectively is limited.